The invention is directed to a method for manufacturing ultraviolet (UV) microscope dry objectives and to microscope objectives manufactured in accordance with this method.
German Patent Publication DE 3,915,868 A1 discloses a dry objective suitable for UV light which has an aperture greater than 0.70 and a magnification of at least 60 times. Correction is provided for aperture aberration for the visible spectral range and also for a specific UV wavelength. However, correction for both the visible spectral range and the UV wavelength at the same time is not provided.
This conventional objective includes at least 11 lenses. At least two lens elements are displaceable relative to one another. The separation of these displaceable lens elements relative to one another determines the wavelength for which the objective is adjusted. Because this separation is set on an outer objective mount, this objective requires an appropriately constructed mechanical mount. If even only a slight radial displacement occurs in the course of axial adjustment of the displaceable lens elements, asymmetric image aberration (coma) occurs over the entire image field. For this reason, the requirements imposed upon the precision of manufacture of this conventional objective are very stringent. Furthermore, when adjusted for UV light, only aperture aberration is corrected, not the aberrations for the larger image field.
As a result of the progressively increasing availability and use of excimer lasers and other lasers emitting short-wavelength UV light having wavelengths between 190 nanometers (nm) and 360 nm there is an increasing need for limited-diffraction-corrected imaging systems for this spectral range. This spectral range of 190 nm to 360 nm will be referred to as the DUV range. Since only a very small number of optical materials which can be used in the DUV range exist, and since in the DUV range the refractive index is very strongly dependent upon wavelength, correction of the color fields over the entire DUV range or even only a significant partial spectrum thereof is not possible. Especially for wavelengths below 300 nm, as a rule only monochromatic objectives or quasi-monochromatic objectives can be constructed. These have a spectral bandwidth on the order of plus or minus one nanometer.
When working at only one discrete wavelength, these restrictions are not necessarily disadvantageous. However, from the standpoint of manufacturing such systems, these restrictions mean that to cover a multiplicity of wavelengths, specifically optimized components must be developed and manufactured for each wavelength and also for various categories of application. For example, in the machining of materials using excimer lasers it is desirable to provide objectives with a large free working space and a small to medium aperture. In contrast, the generation of small structures in the submicrometer range by photolithographic processes and the measurement of such structures requires objectives with the highest possible aperture, and necessarily a small free working space. This requirement for a multiplicity of objectives leads to correspondingly high manufacturing costs.
Thus, there is a real need for an improved method of manufacturing UV microscope dry objectives.